Existentialism and Humanism

Existentialism and humanism is an excellent book by Jean Paul Sartre (isbn 0-413-31300-X). As usual I'm going to quote from a few pages:
There is only one sin, and that's failing to believe you have a choice.
I should be without illusion and I should do what I can.
Many have but one resource to sustain them in their misery, and that is to think, "Circumstances have been against me."
What produces cowardice is the act of giving up.
I cannot obtain any truth whatsoever about myself, except through the mediation of another.
What is not possible is not to choose.
We define man only in relation to his commitments.
The content is always concrete, and therefore unpredictable; it has always been invented.